1. Field of the Invention
The invention relates generally to surveying instruments and more particularly to devices and methods for using reference signals from a satellite navigation receiver to automatically and precisely calibrate electronic distance measurement instruments, and for servo-driving the telescopes in electro-optical total stations.
2. Description of the Prior Art
Electronic distance measurement (EDM) equipment became commercially available after World War-II and has since become very important to the surveying, navigation and scientific communities. Since the introduction of EDM, the instrument size and power consumption have been reduced, and the precision and speed of measurement have been improved. Because the miniaturization of EDM equipment became possible, it made good sense to mount EDM's on theodolites which have telescopes that can precisely sight a horizontal and vertical angle to a target. Such combinations are electro-optical hybrids called "total stations".
Combination electronic theodolite and EDM instruments allow surveyors to find the "space vector" from the instrument to a distant target. When a total station is connected to an electronic data recorder, field information can be quickly gathered and used to generate maps and plans in the office.
Flexible tapes, leveling staves, electro-optical distance meters, and other surveying equipment are calibrated to a legal standard and calibration certificates are issued, e.g., a "Regulation 80 Certificate", as is issued in Western Australia. Such calibration is especially important where a legal purpose is in mind, e.g., an inspection to enforce a law or to be used as evidence in a court action. A flexible tape calibration laboratory in Midland is registered by the National Standards Commission of Australia for calibration of 1-100 meter lengths.
There are two certified baselines in Western Australia against which EDM instruments can be calibrated. The aim of EDM calibration is to ensure that it measures in accordance with the internationally recognized definition of length, as set forth by the Conference Generale des Poids et Measures (CGPM--the General Conference on Weights and Measures). Other governments in the world provide similar baselines and certification opportunities. When a Regulation 80 Certificate is required for the purpose of legal traceability to the Australian Standard for length, the EDM instrument is submitted to the Surveyor General for calibration. The Director of the Mapping & Survey Division is the verifying authority for length and is appointed by the National Standards Commission. The Surveyor General now provides a software application program, called BASELINE, to assist surveyors with their regular calibrations of EDM instruments.
The accuracy of electronic distance measurement equipment is derived from an internal reference frequency source, e.g., a crystal oscillator. But such crystal oscillators can drift over time and with age. Exposure to extreme environments can also upset delicate calibrations of the reference frequency source, both short term and long term. Therefore, EDM equipment should be regularly calibrated by using it to measure a known length.
Long-range electronic distance meters, e.g., ranges over five kilometers, typically use microwave signals for measurement. Short range electronic distance meters often use infrared light. See, Rueger, J. M., Electronic Distance Measurement-An Introduction, Springer Verlag, Berlin, third edition, 1990. Both the long-range and short-range EDM's use pulse or phase comparison methods to determine the distance between instrument and a remote target. However, the phase comparison method is more commonly used for survey instruments.
The pulse technique is based on timing the signal travel time to and from a distant reflector. The velocity of the signal is assumed to be known. For phase comparison, the phase difference of signals is observed at several frequencies. The unambiguous distance between the target and the instrument is resolved using phase difference observations. But in all cases, the basis for measurement precision depends on the accuracy of the stand-alone reference frequency source.
In U.S. Pat. No. 5,471,218, issued Nov. 28, 1995, Nicholas C. Talbot and Mark Nichols, described a combined satellite positioning/electro-optical total station system. One candidate satellite positioning system that can be used effectively is the Global Positioning System (GPS) operated by the United States. Such patent is incorporated herein by reference.
The combined satellite positioning/electro-optical total station system allows rapid instrument orientation and positioning in the field. Another integrated surveying system that combines electro-optical instrumentation with a satellite position measuring system is described by Ingensand, et al., in U.S. Pat. No. 5,233,357.
EDM measurements ultimately rely on the calibration of the instrument. Such calibrations can drift over time, temperature, and mechanical abuse. In general, the law in various jurisdictions specifies a maximum period that an EDM can be operated by a surveyor before its calibration must be certified again. It can also be advantageous in various legal situations where the question is raised on the survey method if the raw measurements are kept in a time-ordered log so that reconstructions of the survey can be performed.